Catalytic reforming is a well established refinery process for improving the octane quality of naphthas or straight run gasolines. Reforming can be defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes, and dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcyclopentanes to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst. In catalytic reforming, a multifunctional catalyst is usually employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, usually platinum, substantially atomically dispersed on the surface of a porous, inorganic oxide support, such as alumina. The support, which usually contains a halide, particularly chloride, provides the acid functionality needed for isomerization, cyclization, and dehydrocyclization reactions.
Reforming reactions are both endothermic and exothermic, the former being predominant, particularly in the early stages of reforming with the latter being predominant in the latter stages. In view thereof, it has become the practice to employ a reforming unit comprised of a plurality of serially connected reactors with provision for heating of the reaction stream from one reactor to another. There are three major types of reforming: semiregenerative, cyclic, and continuous. Fixed-bed reactors are usually employed in semiregenerative and cyclic reforming, and moving-bed reactors in continuous reforming. In semiregenerative reforming, the entire reforming process unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by coke deposition, until finally the entire unit is shut-down for regeneration and reactivation of the catalyst. In cyclic reforming, the reactors are individually isolated, or in effect swung out of line, by various piping arrangements. The catalyst is regenerated by removing coke deposits, and then reactivated while the other reactors of the series remain on stream. The "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, which is then put back in the series. In continuous reforming, the reactors are moving-bed reactors, as opposed to fixed-bed reactors, with continuous addition and withdrawal of catalyst. The catalyst is regenerated in a separate regeneration vessel.
In reforming, sulfur compounds, even at a 1-2 ppm level contribute to a loss of catalyst activity and C.sub.5 + liquid yield, particularly with the new sulfur-sensitive multimetallic catalysts. For example, a platinum-rhenium catalyst is so sensitive to sulfur poisoning that it is necessary to reduce sulfur to well below 0.1 wppm to avoid excessive loss of catalyst activity and C.sub.5 + liquid yield.
Generally, all petroleum naphtha feeds contain sulfur. Consequently, most of the sulfur is usually removed from the feed by hydrofining with conventional hydrodesulfurization catalysts comprised of molybdenum with nickel or cobalt, or both, on a carrier such as alumina. The severity of the hydrofining can be increased so that essentially all of the sulfur is removed from the naphtha in the form of H.sub.2 S. However, small quantities of olefins are also produced. As a consequence, when the exit stream from the hydrofiner is cooled, sulfur can be reincorporated into the naphtha by the combination of H.sub.2 S with the olefins to produce mercaptans. Hence, if a refiner is willing to pay the price, a hydrofining process can be employed at high severity to remove substantially all of the sulfur from a feed, but it is rather costly to maintain a product which consistently contains less than about 1-2 parts per million by weight of sulfur. Also, during hydrofiner upsets, the sulfur concentration in the hydrofined product can be considerably higher, e.g., as high as 50 ppm, or greater.
While hydrofining may remove most of the sulfur from the feedstock, sulfur still remains a problem in catalytic reforming because another source of sulfur results from catalyst presulfiding. It is generally necessary to passivate the active metal sites on fresh, or freshly regenerated catalysts prior to contacting with feed. This helps prevent excessive demethylation reactions, low liquid yields, and possible temperature run-aways. Passivation is accomplished by first reducing the catalyst with hydrogen, followed by treating it with about 0.1 wt. % sulfur in the form of H.sub.2 S, di-teriary polysulfide (TNPS), or other suitable sulfur compounds, particularly the organic sulfur compounds. While most of this sulfur is gradually depleted from the catalyst during normal operation of the unit and removed during removal of make fuel gas, the remainder (up to about 30% or original) is recirculated. In cyclic reformers, this remaining recirculating sulfur has the effect of depressing activity in all of the reactors.
Various techniques have been used to remove sulfur, primarily from the feed. For example, one method for removing sulfur from feedstreams which has met with a limited amount of success is taught in U.S. Pat. No. 4,634,515, which is incorporated herein by reference. This patent teaches removal of sulfur from liquid phase feedstreams by use of a fixed bed of massive nickel catalyst, the nickel being supported on alumina. This method requires use of temperatures in the range of about 300.degree. F. to 500.degree. F. While such a method does in fact remove the sulfur inherent in the feedstock, it does not teach removal of sulfur resulting from presulfiding the catalyst. U.S. Pat. No. 4,519,829 is an improvement on this method, by incorporating, with the massive nickel, from 1 to 15 weight percent iron to suppress the production of PNAs.
Various techniques have also been proposed to remove sulfur from gas streams which could be employed on the recycle gas streams. For example, it has been proposed to remove sulfur by use of zinc alumina spinel, see U.S. Pat. Nos. 4,263,020 and 4,690,806. The drawback of using spinel compositions is that they have a relatively low capacity for sulfur, e.g. 1-2%, and thus, require their own regeneration facility. It has also been proposed to use zinc traps, such as a zinc oxide trap, see for example U.S. Pat. Nos. 4,717,552; 4,371,507; and 4,313,820. Zinc oxide traps tend to deteriorate rapidly in the presence of chloride and thus a chloride trap upstream of the zinc trap is required.
Other references teach the use of various high temperature traps, such as U.S. Pat. No. 4,187,282 which teaches the use of iron/copper/titanium oxide at a temperature from about 480.degree. to 932.degree. F; U.S. Pat. No. 4,273,748 which teaches the use of dual iron/nickel oxide beds operating at temperatures of 842.degree. and 1300.degree. F; and U.S. Pat. No. 4,140,752 which teaches the use of vanadium, nickel, and/or potassium on activated carbon.
While some of the above methods for removing sulfur have met with various degrees of commercial success, there is still a need in the art for the removal of sulfur which is both inherent in the feedstock as well as sulfur resulting from presulfiding the catalyst.